The present invention relates to a hip prosthesis, More specifically, the invention concerns a prosthesis permitting avoiding rotation or sinking phenomenons, thus permitting an optimum diaphyseal blocking.
As it is well known, a hip prosthesis is typically comprised of a femur stem, to be inserted within the medullary channel of the femur on which prosthesis must be provided, comprising articulation of the interested hip, a prosthetic neck, at the end of which a head is inserted, which is part of the prosthetic device and is destined to be inserted within a cavity usually comprised of polyethylene. The head is suitable to be placed within the cotyloid cavity of the hip iliac bone, at the level of which it is usually inserted a metallic cotyle having a suitable shape.
An example of a hip prosthesis according to known technique is described in the US Patent Application no. US 2006/0190092.
A further example of known prosthesis is described in European Patent EP 0 985 385 B1, wherein the femur stem has longitudinal reliefs provided with a particular spacing in order to permit a better fixing. However, said reliefs have a cross-section not permitting an optimum osteo-integration.
One of the main problems of hip prostheses according known art is detachment of femur stem, or the removal of bone capital.
These limits make it difficult to permit possible revisions and maintenance of the same prosthesis; furthermore, standard length of femur stems complicates their use with mini-invasive techniques of treating muscle tissues, tendons and ligaments.
Moreover, standard prosthesis, if shortened, do not fully eliminate serious phenomenons of prosthesis rotation within the femur channel and stem sinking, that must be prevented following the implantation of the same prosthesis.
A further limitation of the known prosthesis is that they do not permit introduction within a medullary channel, since greater trochanter base must be removed, thus reducing preservation of bone tissue. Moreover, as it is well known, the femur is subjected to a bending moment on its proximal portion. Therefore, during the post-implantation step, in case stems are introduced with a recto or acute angle, it is necessary, for surgical reasons, to insert the femur stem after having carried out osteotomy of the femur neck and shoulder. This creates a discontinuity of the bone/prosthesis structure, since a space is created close to the contact point with the prosthesis.
It is also known that femur channel does not have a perfectly circular cross-section, but rather an oval cross-section, with a narrower or wider curvature is provided in the medial position, while larger curvature is provided in lateral position. The broken trapezoidal cross-section usually used for known femur stem, has the advantage of increasing stereo-stability, but has three main drawbacks: (i) its shape imposes four point contacts (always on the cross-section) with femur cortical bone; (ii) stem volume is often under-dimensioned with respect to femur channel; (iii) length of this kind of stem is usually too long, in order to increase its stability.
The circular section stems too are not anatomically suitable to the femur channel shape.